1. Field of the Invention
The present invention relates to an optical scanning apparatus which is used, e.g., in a laser beam printer and the like, more particularly, to an optical scanning apparatus which scans a plurality of pencils of light (referred to as a beam herein after) all at once.
2. Description of the Related Art
In an optical scanning apparatus used, e.g., in a laser beam printer, an optical spot is formed by emitting a beam (laser beam and the like) on the surface of a photosensitive material, and the optical spot is shifted (i.e., scanned) on the surface of the photosensitive material and the like, for example. In such an optical scanning apparatus, a beam emitted from a light source such as a semiconductor laser device is converted into a parallel beam through a collimator lens and the parallel beam is reflected onto a target surface to be scanned (i.e., the surface of the photosensitive material and the like) by a deflecting means such as a polygon mirror.
In recent years, an optical scanning apparatus which scans a plurality of beams has been developed. In such an optical scanning apparatus, a plurality of beams is emitted from the respective light sources aligned, and each of the plurality of beams is converted into a parallel beam via a common collimator lens and then reflected onto the target surface by a common deflecting means. All the beams emitted from any light source have the same distance from the light source to the target surface.
In such an optical scanning apparatus, a plurality of beams passes a common collimator lens. When three or more beams are emitted, there exists a beam emitted from a light source near the optical axis of the collimator lens and a beam emitted from a light source away from the optical axis of the collimator lens. Thus, image points formed with these beams are different due to the curvature of field of the collimator lens in general. Specifically, the image point formed with the beam emitted from the light source farther from the optical axis of the collimator lens is located closer to the collimator lens than that formed with the beam emitted from the light source closer to the optical axis. Such variations in the positions of image formation result in variation in size and shape of the optical spot formed on the target surface. Specifically with recent demand for high-resolution, such variations have become a problem of great difficulty.
To solve such a problem, the curvature of field of the collimator lens has to be reduced. However, reduction of the curvature of field requires the collimator lens to be composed of a plurality of lenses, which causes an increase in manufacturing cost.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide an optical scanning apparatus in which low manufacturing cost is realized as well as images are formed precisely on the target surface with a plurality of beams.
An optical scanning apparatus for scanning a beam on a target surface to be scanned comprises: three or more light sources, each of which emits a beam; a first optical system provided in common with three or more beams emitted from the light sources; deflecting means which deflects the beams passed through the first optical system in a manner that the beams are scanned on the target surface along a scan direction; and a second optical system which forms images on the target surface with the beams deflected by the deflecting means. In the optical scanning apparatus at least one of a first optical path of each beam and a second optical path of each beam is set in accordance with a curvature of field of the first optical system, the first optical path being from each of the light sources to the first optical system and the second optical path being from the deflecting means to the target surface.
In the optical scanning apparatus of the invention, at least one of the first optical path and the second optical path is set in accordance with a curvature of field of the first optical system. Therefore, even though the first optical system has the curvature of field, all of the image points formed with the beams are located just on the target surface.
Preferably, the first optical system includes a collimator for converting each of the beams into a parallel beam in substance, and at least one of the first optical path and the second optical path is set in accordance with a curvature of field of the collimator. Preferably the first optical system further includes a convergent optical system which converges each of the beams, converted into the parallel beam in substance by the collimator, only in a direction corresponding to a direction orthogonal to the scan direction on the target surface, so that each of the beams is focused in the vicinity of the deflecting means.
Preferably, the larger a distance between a light source and an optical axis of the first optical system is, the shorter at least one of the first optical path and the second optical path of the beams emitted from the light source is. Further, the optical path along each beam from the deflecting means to the target surface may be changed by means of changing a configuration of the second optical system. Alternatively, the optical path along each beam from the deflecting means to the target surface may be changed by means of adjusting a position of the target surface. Preferably, the second optical system has a common part through which all of the beams deflected by the deflecting means pass, the common part having a converging ability only in the scan direction. Preferably, the second optical system has a common part through which all of the beams deflected by the deflecting means pass, and center axes of the beams which pass through the common part are parallel to each other.
Other and further objects, features and advantages of the invention will appear more fully from the following description.